refactor: move EspIdfHal.h to src/helpers/radiolib/ per reviewer suggestion

Move the ESP-IDF SPI HAL from the variant-specific directory to the
shared helpers/radiolib/ directory, making it available for reuse by
other ESP32 variants that may need the same SPI workaround.

Addresses feedback from @ripplebiz: 'I'd probably move the IDF Hal to
/src/helpers/radiolib, as it's related only to RadioLib and could
potentially be re-used by other variants'

Also update the include path in target.cpp to use the shared location.

variants/nicerf_lr2021/target.cpp

@@ -8,7 +8,7 @@ NiceRFLR2021Board board;
 // your board, define USE_ESPIDF_HAL in platformio.ini to use the ESP-IDF SPI
 // HAL workaround (see EspIdfHal.h for details).
 #ifdef USE_ESPIDF_HAL
-#include "EspIdfHal.h"
+#include <helpers/radiolib/EspIdfHal.h>
 static EspIdfHal hal(P_LORA_SCLK, P_LORA_MISO, P_LORA_MOSI);
 RADIO_CLASS radio(new Module(&hal, P_LORA_NSS, P_LORA_DIO_1, P_LORA_RESET,
                              P_LORA_BUSY));